Artificial leathers have come to be widely used in clothes, general materials, sport goods, material for bags, etc. because its superiority to natural leathers, such as light weight and easiness of handling, has been accepted by consumers.
Recently, consumer's tastes have been broadened and artificial leathers added with values come to be preferred. For example, consumers need artificial leathers having high quality appearance with gloss. For such artificial leathers, pearl artificial leathers have been known. For example, Patent Document 1 discloses a nubuck artificial leather made of a foamed polyurethane containing metallic powder.
However, the artificial leather proposed in Patent Document 1 loses the gloss during its long use because the metallic powder is merely coated on the surface and therefore easily drops off from the surface.
The method of producing artificial leathers generally used roughly includes a step in which microfine fiber-forming fibers made of two kinds of polymers having different solubility are made into staple fibers, a step in which the staple fibers are formed into a web by using a carding machine, crosslapper, random webber, etc., a step in which the fibers are entangled to one another by a needle-punching, etc. to form an entangled non-woven fabric, a step in which a solution of an elastic polymer such as polyurethane is impregnated into the entangled non-woven fabric, and a step in which the microfine fiber-forming fibers are converted into microfine fibers by removing one of components in the composite fibers.
However, the easy pull-out and drop-off of the staple fibers from the non-woven fabric are inevitable because of their short fiber length. With such drawbacks, the important surface properties, such as the abrasion resistance of the napped surface of suede-finished artificial leather and peeling strength of grain-finished artificial leather, are insufficient. In addition, the resulting product is poor in dense feeling, surface appearance, and quality stability because the fabric is excessively elongated and fibers on the surface are pulled out during its production.
Unlike the production of a staple nonwoven fabric, the production of a filament nonwoven fabric is simple because a series of large apparatuses such as a raw fiber feeder, an apparatus for opening fibers and a carding machine is not needed. In addition, the filament nonwoven fabric is superior to the staple nonwoven fabric in the strength and shape stability. The attempt to use a filament web as the substrate of artificial leather has been made. However, only a grain-finished artificial leather having a substrate which is made of filaments having a normal fineness of 0.5 dtex or more has been on the market. Artificial leathers made of microfine filament have not yet been put on the market. This is because that an entangled web having a stable mass per unit area (a stable weight) is difficult to produce from filaments, the uneven fineness and strain of composite filaments likely cause uneven product quality, and the dense feeling is poor and the hand likely becomes cloth-like because filaments are poor in bulkiness as compared with crimped staples.
To prevent the unevenness and improve the bulkiness, a method of partly relieving the strain by partly cutting filaments so as to densify the web (for example, Patent Document 2). Patent Document 2 describes that the strain markedly caused during the entangling treatment of filaments can be relieved by intentionally cutting the filaments during the entangling treatment by needle punching, thereby exposing the cut ends of filaments to the surface of the nonwoven fabric in a number density of 5 to 100/mm2. The document further describes that 5 to 70 fiber bundles are present per 1 cm width on the cross section parallel to the thickness direction of the nonwoven fabric of filaments, i.e., the number of fiber bundles which are oriented by needle punching in the thickness direction is 5 to 70 per 1 cm width of the cross section. The document further describes that the total area of fiber bundles on a cross section perpendicular to the thickness direction of the nonwoven fabric of filaments is 5 to 70% of the cross-sectional area. Although cutting the filaments to an extent achieving the intended properties, many filaments are required to be cut to make the nonwoven fabric of filaments into the proposed structure. Therefore, the advantages of using filaments that the strength of nonwoven fabric is enhanced because of their continuity are significantly reduced, thereby failing to effectively use their advantages. To cut the fibers on the surface of nonwoven fabric evenly, the filaments should be entangled by repeating the needle punching many times under conditions severer than usual, thereby making it difficult to obtain a nonwoven filament fabric of high quality and high strength aimed in the present invention.
In another proposed method, a filament web with a good flatness, smoothness and hand is obtained by hot-pressing a web of dividable composite filaments (spun-bonded fleece) at high temperature to bond filaments for controlling the shrinkability and then conducting punching treatment first by using needles (No. 1) having a barb depth of 3 to 10 times the fiber diameter and then by using needles (No. 2) having a barb depth of 1 to 6 times the fiber diameter (for example, Patent Document 3). This method is effective for simultaneously conducting the entanglement and the division of fibers while moderately cutting the dividable composite filaments. However, since the filaments are cut, the deterioration of properties of the non-woven fabric is inevitable. In this method, before needle-punching, the spun-bonded fleece is heat-treated by a calendar roll to control the shrinkability of filaments, improve the conveying ability, and control the hand and density of final products. However, since the heat treatment conditions are determined according to the intended shrinkage, it is practically difficult to control the degree of fuse-bonding of filaments on the surface of the spun-bonded fleece, because the dividable filaments have a multi-layered structure of the components having different meting points.    Patent Document 1: JP Patent No. 3056609    Patent Document 2: JP Patent No. 3176592    Patent Document 3: JP 2005-171430A